An electric power steering apparatus (EPS) which provides a steering system of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a motor driving force controlled with an electric power supplied from an inverter as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty ratio of pulse width modulation (PWM) control. A brushless motor that is superior in durability and serviceability and has little noise, is commonly used as the motor.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3 in a reducing section, universal joints 4a and 4b, a rack and pinion mechanism 5, tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Ts of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angel θ, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command based on the steering torque Ts detected by the torque sensor 10 and a vehicle speed Vs detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 for EPS based on a voltage command value Vref obtained by performing compensation and so on with respect to the current command value.
Further, the steering angle sensor 14 is not indispensable and may not be provided, and it is possible to obtain the steering angle from a rotational position sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 40 to exchanging various information of a vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vs from the CAN 40. Further, it is also possible to connect a non-CAN 41 exchanging a communication, analog/digital signals, a radio wave or the like except the CAN 40 to the control unit 30.
The control unit 30 mainly comprises a CPU (including an MCU, an MPU and so on), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Ts detected by the torque sensor 10 and the vehicle speed Vs detected by the vehicle speed sensor 12 (or from the CAN 40) are inputted into a current command value calculating section 31 calculating a current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1 that is a control target value of a current supplied to the motor 20 based on the steering torque Ts and the vehicle speed Vs that have been inputted and by means of an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A. A current command value Irefm the maximum current of which is limited is inputted into a subtracting section 32B, and a deviation I (Irefm−Im) between the current command value Irefm and a motor current value Im being fed back is calculated. The deviation I is inputted into a PI-control section 35 for improving a characteristic of the steering operation. The voltage command value Vref whose characteristic is improved by the PI-control section 35 is inputted into a PWM-control section 36. Furthermore, the motor 20 is PWM-driven through an inverter 37 serving as a driving section. The motor current value Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtracting section 32B. The inverter 37 uses field effect transistors (FETs) as driving elements and is comprised of a bridge circuit of FETs.
A rotational position sensor 21 such as a resolver is connected to the motor 20, and a motor rotational angle θe is outputted from the rotational position sensor 21.
A compensation signal CM from a compensation signal generating section 34 is added to the adding section 32A, and a characteristic compensation of the steering system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation signal generating section 34 adds a self-aligning torque (SAT) 343 and an inertia 342 in an adding section 344, further adds the result of addition performed in the adding section 344 with a convergence 341 in an adding section 345, and then outputs the result of addition performed in the adding section 345 as the compensation signal CM.
In such an electric power steering apparatus, cases that use a motor having a multi-system motor winding of a constitution to continue a motor operation even if a failure (including an abnormality) of the motor occurs, are increasing. For examples, with respect to a motor having two-system motor windings, coils of a stator are separated into two systems (a U1 phase to a W1 phase and a U2 phase to a W2 phase), and it is possible to rotate a rotor of the other system even if the failure occurs in one system and to continue an assist control.
With respect to an electric power steering apparatus equipped with such a motor, when operating normally, two systems generally rotate the rotor cooperatively, and current command values for respective motor windings are distributed (a half of the current command value are distributed respectively when characteristics of motor windings of two systems are equivalent). Therefore, when an abnormality occurs in one system, a current command value for a motor winding of the other normal system remains as the distributed value if no change is performed, so that there is the possibility that a steering assist torque decreases. When a compensation signal is added to the current command value, the compensation signal also becomes small when the abnormality occurs, so that the characteristic compensation for improving the convergence, the inertia characteristic and so on is not performed sufficiently, a vibration increases because a stability is deteriorated, and this may causes deterioration of steering feeling.
With respect to such deterioration of steering feeling in the case that the abnormality occurs in motor control by the multi-system, a method to suppress the deterioration has been proposed. For example, in Japanese Unexamined Patent Publication No. 2015-39256 A (Patent Document 1), a motor control apparatus is proposed that can continue to drive and control a motor even if an open failure or a short failure occurs in a motor driving circuit, by comprising a motor current cut-off section interposed between the motor driving circuit and a motor winding, an abnormality detecting section that detects an abnormality of a motor drive current or a motor drive voltage, and so on.